The innermost region of AGN tori:
implications from the HST/NICMOS Type 1 point sources and near-IR
reverberation
Kishimoto, M., Hönig, S., Beckert, T.,
and Weigelt, G.
A&A, 476, pg.713-721 (2007)
Abstract
Spatially resolving the innermost region of the putative torus-like
structure in an active galactic nucleus (AGN) is one of the main
goals of its high-spatial-resolution studies. This could be done in the
near-IR observations of Type 1 AGNs where we see directly the
hottest dust grains in the torus.We discuss two critical issues in such
studies. One is the possible contribution from the central putative
accretion disk (the near-IR part of the big blue bump emission), which
should be taken into account for the torus measurements. The
other is the expected size of the inner boundary of the torus,
essential for the feasibility of spatially resolving the region.
Firstly, we examine the nuclear near-IR point sources in the HST/NICMOS
images of nearby Type 1 AGNs, to evaluate the accretion
disk contribution. After the subtraction of the host bulge flux through
two-dimensional decompositions, we show that near-IR colors
of the point sources appear quite interpretable simply as a composite
of a black-body-like spectrum and a relatively blue distinct
component as expected for a torus and an accretion disk in the near-IR,
respectively. The near-IR colors of our radiative transfer
models for clumpy tori also support this simple two-component
interpretation. The observed near-IR colors for the available sample
suggest a fractional accretion disk contribution of ∼25% or less at 2.2
µm.
Secondly, we show that the innermost torus radii as indicated by the
recent near-IR reverberation measurements are systematically
smaller by a factor of ∼3 than the predicted dust sublimation radius
with a reasonable assumption for graphite grains of sublimation
temperature 1500 K and size 0.05 µm in radius. The discrepancy might
indicate a much higher sublimation temperature or a typical
grain size being much larger in the innermost tori, though the former
case appears to be disfavored by the observed colors of the HST
point sources studied above. Alternatively, the central engine
radiation might be significantly anisotropic. The near-IR
interferometry
with a baseline of ∼100 m should be able to provide the important,
independent size measurements for the innermost torus region,
based on the low fractional contribution from the accretion disk
obtained above.
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